The present invention relates to an electrodeless discharge lamp apparatus using microwaves.
An electrodeless discharge lamp has no electrode inside a discharge space, and therefore blackening on the inner wall of a bulb due to evaporation of electrodes does not occur. Thus, it is possible to prolong the lamp life significantly. With this feature, electrodeless discharge lamps have been under in-depth research as the next generation high-intensity discharge lamp in recent years. In discharge lamp apparatuses in general, as the light emitting portion is smaller, the lamp is closer to a point light source and thus ideal luminous intensity distribution can be designed. Therefore, there is strong demand for reduction in the size of plasma, which is a light emitting portion.
In the case of an electrodeless discharge lamp apparatus using microwaves (microwave-excited lamp apparatus), microwaves are generated by magnetron and are passed through a wave guide to cause discharge in an electrodeless discharge lamp in a cavity resonator for light emission. In the case of this lamp apparatus, the minimum size of the cavity resonator is, in principle, determined by the frequency of the microwaves. For an electrodeless discharge lamp using microwaves of 2.45 GHz (wavelength of 122 mm), which is commonly used, it is known empirically that the size of a plasma arc that can maintain stable discharge is limited to about 15 mm or more. This size of the plasma arc is far from the size of the plasma arc that can be designed as being regarded as a point light source (e.g., 3 mm or less) in the optical design.
In the electrodeless discharge lamp apparatus using microwaves, a technique disclosed in Japanese Laid-Open Patent Publication No. 10-189270 is known that can realize a small sized light-emitting portion. Hereinafter, the electrodeless discharge lamp apparatus disclosed in this publication will be described with reference to FIG. 10.
FIG. 10 schematically shows the structure of high frequency energy supplying means that is a component of the electrodeless discharge lamp apparatus disclosed in this publication. The high frequency energy supplying means shown in FIG. 10 includes a plurality of side resonators and supplies microwave energy necessary for discharge by a resonant microwave electric field in the center of the ring of the side resonators. This structure allows the microwave resonant electric field to be supplied while being concentrated on a space smaller than when using a cavity resonator.
The high frequency energy supplying means shown in FIG. 10 is a vane-type resonator, and this vane-type resonator has a structure in which four plate-like vanes 52 made of conductive material extend toward the center from the surface of the inner wall of a member 53 that serves as a reflecting mirror and also serves as a shield for preventing leakage of microwaves. The member 53 is made of conductive material as well and has a circular and rotationally symmetric shape. One of the vanes 52 is joined to a core line of a wave guide 54 by soldering or the like, and thus the vane and the core line are electrically connected so that microwave energy coupling means (microwave coupler) 55 is formed. The microwave energy coupling means 55 acts as an oscillating antenna in the resonator, so that microwave energy propagated through the wave guide 54 is coupled to the vane-type resonator. The size of the vane-type resonator is designed such that resonance occurs at the frequency of the microwave energy to be coupled.
An electrodeless discharge lamp 51 is a lamp in which a luminous material such as a metal halide and a rare gas are enclosed inside a hollow spherical quartz glass. The electrodeless discharge lamp 51 is placed in a microwave resonant electric field generated in the center of the vane-type resonator so that microwave energy is supplied to the electrodeless discharge lamp 51. Thus, discharge is caused by the gas in the electrodeless discharge bulb 51 so that light is emitted. The radiated light due to the discharge is reflected by the reflecting mirror 53 made of a conductor and is released out through a metal net 56. The reflecting mirror 53 in combination with the metal net 56 acts as microwave leakage prevention means.
According to this high-frequency energy supplying means, in the electrodeless discharge lamp, plasma of a comparatively small size of 10 mm or less can be discharged and maintained.
However, as a result of examination of the inventors of the present application, it was found that the system using the side resonators as shown in FIG. 10 has the following problems. First, it is necessary to provide a protruded portion of the side resonators perpendicularly to the central axis of the reflecting mirror with a curved surface, so that even if plasma of a comparatively small size can be discharged and maintained, the structure thereof is complicated. This complication of the structure is detrimental to mass production and increases the cost. Furthermore, in this structure, the light that is radiated toward the reflecting mirror in the direction of the side of the electrodeless discharge lamp is shielded by the protruded portion of the side resonators, and therefore the projected light has shadows of the protruded portion. As a result, problems such as a reduction in the amount of light and non-uniformly distribution of light are caused.
It is an object of the present invention to provide an electrodeless discharge lamp apparatus with a comparative simple structure having excellent luminous intensity distribution properties.
An electrodeless discharge lamp apparatus of the present invention includes a) an electrodeless discharge lamp having no electrode exposed inside a discharge bulb; b) a microwave resonator; and c) a microwave coupler for coupling microwave energy to the microwave resonator. The microwave resonator includes a conductive reflecting mirror having an opening; a conductive shield covering the opening of the reflecting mirror and transmitting light in at least a portion thereof; and two opposing external electrodes provided substantially on the central axis of the reflecting mirror. The electrodeless discharge lamp is disposed between the opposing external electrodes. The focal point of the reflecting mirror is positioned between the opposing external electrodes. When microwave energy is supplied to the microwave resonator via the microwave coupler, a microwave resonant electric field occurs between the opposing external electrodes, whereby discharge of the electrodeless discharge lamp occurs.
Another electrodeless discharge lamp apparatus of the present invention includes a) an electrodeless discharge lamp having no electrode exposed inside a discharge bulb; b) a microwave resonator; c) a microwave coupler for coupling microwave energy to the microwave resonator; and d) a reflecting mirror provided outside the microwave resonator. The microwave resonator includes a conductive cylinder having an opening; a conductive shield covering the opening of the conductive cylinder and transmitting light in at least a portion thereof; and two opposing external electrodes provided substantially on the central axis of the conductive cylinder. The electrodeless discharge lamp is disposed between the opposing external electrodes. The focal point of the reflecting mirror is positioned between the opposing external electrodes. When microwave energy is supplied to the microwave resonator via the microwave coupler, a microwave resonant electric field occurs between the opposing external electrodes, whereby discharge of the electrodeless discharge lamp occurs.
It is preferable that the electrodeless discharge lamp is provided substantially on the central axis of the reflecting mirror and provided substantially on the central axis of the conductive cylinder.
It is preferable that a distance adjuster for adjusting the distance between the opposing external electrodes be provided external to the microwave resonator.
In one preferable embodiment, one of the opposing external electrodes serves also as the microwave coupler.
In one preferable embodiment, said one of the opposing external electrodes is made of a coaxial line, and the microwave coupler is a coaxial core line portion projected from one end of the coaxial line.
In one preferable embodiment, one of the opposing external electrodes serves also as supporting means of the electrodeless discharge lamp.
In one preferable embodiment, a starting probe is provided inside the supporting means.
In one preferable embodiment, the reflecting mirror is of a shape with an ellipsoidal surface.
In one preferable embodiment, a secondary reflecting mirror of a shape with a spherical surface with the electrocleless discharge lamp as the center thereof is further provided in front of the opening of the reflecting mirror, and the secondary reflecting mirror has an opening in a portion in which light is concentrated by the ellipsoidal surface of the reflecting mirror and in the vicinity thereof.
In one preferable embodiment, the electrodeless discharge lamp apparatus further includes cooling means for cooling the electrodeless discharge lamp.
In one preferable embodiment, the electrodeless discharge lamp apparatus includes a wave guide connected to the microwave coupler, wherein the wave guide has a function to propagate microwaves generated by a microwave oscillator.
Since the electrodeless discharge lamp apparatus of the present invention includes an electrodeless discharge lamp, a microwave resonator and a microwave coupler, and the microwave resonator includes two opposing external electrodes provided substantially on the central axis of the reflecting mirror, the present invention can have excellent luminous intensity distribution properties in a comparatively simple structure.